Method and a device for detecting an internal arc in a metal-clad electrical link

ABSTRACT

The invention relates to a method of detecting an internal arc in a metal-clad electrical link, wherein the rising edge of the internal pressure surge generated by the arc and propagating inside the cladding is detected at a fixed location along the cladding. The invention also relates to a device comprising at least one one-way detection unit constituted by a pressure sensor disposed outside the cladding and connected to the inside the cladding via a horn-shaped duct which slopes relative to the axis of the cladding, which has its mouth opening out in the cladding, and which has its tip connected to the pressure sensor.

The present invention relates to a method and a device serving to detectan internal arc in a metal-clad electrical link, in particular for linksthat are long, e.g. 100 meters (m) in length.

BACKGROUND OF THE INVENTION

When a short-circuit current occurs in a metal-clad electrical link, anincrease in the pressure inside the cladding is generated at the fault,and that pressure increase results in two pressure surges beinggenerated, each of which propagates in a respective direction away fromthe place where the short-circuit current has occurred. By detecting thepressure surge, it is possible to observe the appearance of a fault inthe line. It is particularly advantageous to locate the place at whichthe fault occurs so as to detect faults that are repetitive.

Currently, the pressure surge generated by the short-circuit current isdetected by measuring the increase in the mean pressure by means of apressure sensor. For this purpose, it possible to use densitytransmitters that serve to monitor the density of the gas inside thecladding segment in question, each such density transmitter beingconstituted by a pressure sensor whose measurement is compensated as afunction of temperature so as to obtain the density of the gas.

The pressure surge that is generated is relatively small, e.g. about10%, and the pressure surge that propagates inside the cladding from thefault point is attenuated as it propagates and its value decreases goingaway from the fault point. As a result, the sensitivity of thosedetectors is low, and it is not always possible to detect faults thatoccur in segments of long length.

The problem addressed by the invention is to improve detectionsensitivity and to determine the position of the short-circuit arcrelative to the detector.

OBJECTS AND SUMMARY OF THE INVENTION

In the present invention, the rising edge of the internal pressure surgegenerated by the arc and propagating inside the cladding is detected ata fixed location along the cladding.

It is easier to detect the passage of the rising edge of a pressuresurge because its amplitude is greater than the mean pressure variation,and this significantly increases the sensitivity of the sensor.

According to another characteristic of the invention, the pressureinside the cladding is measured continuously for two equal durationsshorter than the time taken by the rising edge to go past, and separatedby a time interval shorter than the time taken by the is rising edge togo past, the curve of the measurements taken during each of saiddurations is integrated, the difference between the two integrationvalues is determined, and the pressure difference obtained is comparedwith a threshold value.

The pressure of the dielectric gas is monitored continuously, and thisalgorithm makes it possible to measure two values of the pressure at twoclose-together instants, and the resulting value is independent of thesteady pressure value which can itself be subject to fluctuations. As aresult, accuracy is significantly improved, and therefore so is thesensitivity with which the passing pressure surge is detected.

Advantageously, the measurement durations are substantially equal to onehalf of the time taken by the rising edge to go past, and the timeinterval is substantially equal to one half of the time taken for therising edge to go past. These times depend on the propagation speed ofpressure waves in the dielectric gas.

This makes it possible to compare pressure values during two durationscorresponding respectively substantially to the beginning of the risingedge and to the end thereof so that the difference between theintegrated values in maximized.

According to another characteristic of the invention, the detection isperformed substantially in the middle of the cladding segment to bemonitored.

In this way, sensitivity is further improved because the attenuation ofthe pressure surge as it propagates is limited to the attenuationcorresponding to one half of the length of the segment to be monitored.

The invention also provides a device for implementing. theabove-mentioned method, the device comprising at least one one-waydetection unit constituted by a pressure sensor disposed outside thecladding and connected to the inside the cladding via a horn-shaped ductwhich slopes relative to the axis of the cladding, which has its mouthopening out in the cladding, and which has its tip connected to thepressure sensor.

The resulting interface between the segment to be monitored and thesensor makes it possible firstly to improve sensitivity because of theadditional surge pressure generated by the duct which constitutes a sortof funnel. Furthermore, the resulting detection unit acts in one-waymanner and is sensitive only to the pressure surges coming from thedirection opposite to the direction in which the duct slopes.

Advantageously, there is provided a set of at least two one-waydetection units having the same detection direction and disposed in thevicinity of each other, and whose ducts elope the same way.

This configuration makes it possible to obtain redundancy in terms offault location by mitigating sensor failure.

It is possible to provide two one-way detection sets whose ducts slopein mutually opposite directions and which are disposed in the vicinityof each other.

In this way, by using four detection units, it is possible to detect andto locate, with good sensitivity and high reliability, faults that occuron either side of the detection sets.

In an embodiment of the invention, use is made of two pressure seniorsconnected in parallel to two ducts which slope in mutually oppositedirections and whose tips communicate with each other.

This configuration makes it possible to form a both-way detector deviceof high sensitivity and having detection redundancy because the sensorsare duplicated.

In another embodiment of the invention, use is made of two directionaldetection units whose ducts slope in mutually opposite directions.

A both-way detection device is thus obtained that makes it possible tolocate the faults but that has no back up against sensor failure.

Advantageously, for these two embodiments, use is made of the twosensors serving to monitor the density of the dielectric gas. This makesit possible to form a detection device at little extra cost.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention appear from thefollowing description given by way of non-limiting example, withreference to the accompanying drawings, in which:

FIG. 1 is a hydraulic analogy explaining the phenomenon of the pressurewave;

FIG. 2 describes the algorithm used;

FIG. 3 shows the amplification obtained by the interface duct;

FIG. 4 shows a both-way detection device using the hydraulic analogy;

FIG. 5 shows an embodiment of such a both-way detection device;

FIG. 6 shows an embodiment of a one-way detection device having twosensors; and

FIG. 7 is a section view of the device of FIG. 6.

MORE DETAILED DESCRIPTION

FIG. 1 shows a hydraulic analogy of the phenomenon created by ashort-circuit current in a metal-clad electrical link, such a link alsobeing referred to as a “metal-clad cable”, the segments of which linkcan be of considerable length, e.g. over 100 m long.

It should firstly be recalled that, over a short period of time, i.e. afew seconds in the present case, the pressure surge may be considered tobe adiabatic since the heat exchange with the outside is negligible oversaid period. As a result, the heat energy contributed by the internalarc current generates an increase in the mean pressure inside thesegment in question.

It is possible to make an analogy with a water channel 1 which, at somelocation along its bottom, is provided with a wide cylinder-and-pistonsystem as shown in FIG. 1a. Raising the piston (FIG. 1b) causes anincrease in the mean level of the water in the channel.

Clearly, if the channel is long, the increase in water level isnegligible and difficult to detect. In analogous manner, it is difficultfor an internal arc to be detected by means of the increase in the meanpressure in a long metal-clad cable.

On each side of the system 2, a wave 3 appears whose amplitude is afunction of the characteristics (dimensions, piston speed) of the system2. The wave propagates away from the location of the system 2 as shownin FIGS. 1c to 1 e.

The amplitude of the variation in the water level at the wave is muchgreater than the variation in the mean water level, and it is thereforemuch easier to detect.

Considering a segment of metal-clad cable, by analogy, a pressure waveor surge is generated that is easier to detect than the variation in themean pressure. This has been verified experimentally by the applicant.

That is why the invention proposes to detect the passage of the pressuresurge, and, more particularly, to detect the rising edge of said surgeas it goes past, which edge is relatively steep and thus generates arapid variation. It is thus possible for the sensitivity with whichinternal arcs are detected to be increased considerably.

FIG. 2 shows the algorithm of a method of detecting said rising edge.With a pressure sensor connected to the cladding, the pressure of thedielectric gas in said cladding is measured continuously. FIGS. 2a and 2b show how the pressure P varies over time t at the sensor 4 due to thepressure surge going past.

This measurement is integrated continuously for two adjacent durationsof 40 ms, namely from t-120 ms to t-80 ms, and from t-40 ms to t. Eachof these two durations is equal to 40 ms and they are separated by atime interval that is also 40 ms in duration. This duration of 40 ms issubstantially equal to one half of the time required for the rising edge5 of the pressure surge 6 to go past the sensor. As shown in FIG. 2b,this makes it possible to measure and to compare the normal value of thepressure for the duration t-120 ms to t-80 ms with a value in thevicinity of the peak pressure for the duration t-40 to t. The differenceΔ between the two integrated values is calculated as follows:

Δ=∫_(t-40) ^(t) Pdt−∫ _(t-120) ^(t-80) Pdt

if this difference Δ, which is positive while the rising edge 5 of thepressure surge 6 is going past, is greater than a threshold value ΔP,which is shown in FIG. 2b and which is slightly less than the minimumpossible value of the pressure surge generated by the internal arc, thenit is concluded that an internal current fault has occurred in themetal-clad cable segment on which the pressure sensor is placed.

As shown in FIG. 3, according to another characteristic of theinvention, for the purpose of further improving the sensitivity of themeasurement, the sensor 4 is connected to the cladding via a duct 7which, to return to the hydraulic analogy, is of shape and directionorganized to form a “scoop” that receives the pressure surge.

The duct 7 is horn-shaped and constitutes a funnel. The mouth 8 of theduct 7 is connected to the cladding of the segment to be monitored, andits tip 9 is connected to the pressure sensor.

As shown in FIG. 3, the surge pressure difference is shown in FIG. 3a isamplified by the duct 7 because of the pressure increase generated bythe tapering of the section of the duct 7 (FIG. 3b). It is thus possibleto determine a detection threshold ΔP which is approximately in therange 100 mb to 300 mb in the case of a metal-clad cable containingsulfur hexafluoride. This threshold value is determined as a function ofthe dielectric gas used and of its density inside the cladding.

FIG. 4 shows a set of two adjacent one-way-detection units, each ofwhich is constituted by a duct 7, 7′ connected to a sensor 4, 4′. Thetwo ducts 7 and 7′ slope in mutually opposite directions relative to theaxis of the cladding 11, duct 7 sloping rightwards and duct 7′ slopingleftwards, so that each of the sensors 4 and 4′is sensitive only topressure surges coming from the direction opposite to its direction ofslope, sensor 4 being sensitive to pressure surges coming from the left,and sensor 4′ being sensitive to pressure surges coming from the rightin FIG. 4. This configuration makes it possible to locate the faultrelative to the one-way detection units.

In addition, if the instant at which the internal arc appears in knownby means of grid protection apparatus, then the distance between theplace at which the internal arc appears and the one-way detection unitcan be determined merely by measuring time, the propagation speed of thepressure surge being known for the dielectric gas used. This makes itpossible to locate the fault accurately.

Advantageously, the one-way detection units are disposed in the centerof the segment to be monitored, thereby making it possible to limit theeffect of the pressure surge being damped as it propagates and,therefore, to improve the sensitivity for a long-length segment to bemonitored.

The two sensors are advantageously also constituted by the sensors usedto perform leakage monitoring by measuring the density of the dielectricgas. The device shown in FIG. 4 therefore involves no significant extracost.

FIG. 5 diagrammatically shows a set of two one-way detection unitspositioned on a metal-clad cable containing a central conductor 10disposed in cladding 11 filled with a dielectric gas such as sulfurhexafluoride (SF₆) under pressure. The two ducts 12 and 13 open out intothe cladding 11 and they are angularly positioned in oppositedirections, Each of them is connected to a pressure sensor of known type14, 15, such as those used to monitor the density of the dielectric gas.

For redundancy purposes, it is advantageous to duplicate the one-waydetection units by using four units disposed at the same place, namelytwo left detection units and two right detection units. In this manner,it is possible to mitigate the consequences of any one unit failing, andin particular of a sensor failing, and to increase the reliability ofthe system. If the availability requirements are very high, it ispossible to triplicate the one-way detection units,

FIGS. 6 and 7 show another embodiment of the invention, in whichembodiment two opposite-direction ducts 20 and 21 are provided whosetips 22 and 23 are connected together and are connected to two sensors24 and 25 disposed in parallel. As in the embodiment shown in FIG. 5,the two sensors are those already installed on the cladding 11 for thepurpose of monitoring the density of the dielectric gas, and, at 26, afiller valve can be seen that makes it possible to compensate for anydensity losses that are detected.

The above description is given merely by way of non-limiting example,and, naturally, it is possible to make modifications thereto or toprovide variants thereof without going beyond the ambit of the inventionas defined in the claims. In particular, the value of the duration forwhich the pressure is measured is a function of the propagation speed ofpressure surges in the dielectric gas; thus, in the case of anSF₆-nitrogen mixture, said duration is shorter because the propagationspeed is higher than in SF₆.

What is claimed is:
 1. A method of detecting an internal arc in ametal-clad electrical link, comprising detecting, at a fixed locationalong a cladding of the electrical link, a rising edge of an internalpressure surge generated by the internal arc and propagating inside thecladding, wherein the detecting of the rising edge comprises measuring apressure inside the cladding continuously for first and second timeintervals which are equal in duration, shorter than a time taken by therising edge to go past the fixed location and separated by a third timeinterval shorter than the time taken by the rising edge to go past thefixed location, integrating a curve of the pressure measurements takenduring each of the first and second time intervals, determining adifference between the two integration values, and comparing thedifference with a threshold value.
 2. A method according to claim 1,wherein the first and second time intervals are substantially equal toone half of the time taken by the rising edge to go past the fixedlocation.
 3. A method according to claim 2, wherein the third timeinterval is substantially equal to one half of the time taken for therising edge to go past the fixed location.
 4. A method according toclaim 1, wherein the fixed location is substantially in the middle ofthe cladding segment to be monitored.
 5. A device detecting an internalarc in a metal-clad electrical link, the device comprising at two-waydetection unit for detecting a rising edge of an internal pressure surgegenerated by the internal arc and propagating inside a cladding of theelectrical link, the detection unit including first and second pressuresensors disposed outside the cladding, and first and second taperedhorn-shaped ducts connected to the inside of the cladding and slopingmutually opposite directions relative to the axis of the cladding,wherein the first and second tapered horn-shaped ducts have mouthsopening in the cladding, and tips which joined together at a commonjunction, and said first and second pressure sensors are connected inparallel to the common junction.
 6. A device according to claim 5,further comprising a filler valve connected to the common junction forcompensating density losses.